This project concerns the physiology of the hypothalamic-pituitary-thyroid axis in animals and man. Current studies are directed towards; 1) The development and validation of a radioimmunoassay for 3, 31,51-triiodothyronine (reverse T3); 2) the mechanisms by which various illnesses reduce serum triiodothyronine(T3) and raise serum reverse T3 concentrations and the effects thereon on TRH and TSH secretion; 3) the conversion of thyroxine(T4) to T3 in liver cells and the nature and regulation of this process; 4) the development of methods for recognition of "remission" in patients with Graves' disease using immunoglobulin receptor or thyroglobulin assays; 5) the development of simple methods for measurement of thyrotropin-releasing hormone(TRH) in biological fluids; and b) investigation into the role of TRH in neural function. BIBLIOGRAPHIC REFERENCES: Winokur, A., and R.D. Utiger. The subcellular distribution of thyrotropin-releasing hormone in rat hypothalamus and brain. In Thyroid Research, J. Robbins and L. Braverman, eds. Excerpta-Medica, New York, 1976, pp. 5-7. Buerklin, E.M., M. Schimmel, and R.D. Utiger. Pituitary-thyroid regulation in euthyroid patients with Graves' disease previously treated with antithyroid drugs. J. Clin. Endocr. Metab. 43:419-427, 1976.